Context: Recently, the Institute for Astronomy at the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS) revealed that a new asteroid with a comet-like-tail has been discovered moving in the same orbit travelled by Jupiter around the sun.
Key Points

• A very rare type of space object, which is something between an asteroid and a comet, has been discovered by astronomers. It has been called an Active Asteroid.
• This is the first Jupiter Trojan astronomers have ever seen spewing out gas as a comet would.
• The newly discovered asteroid named 2019 LD2 has an orbit like an asteroid but the tail of a comet.
• While not entirely unknown, what makes 2019 LD2 special is that it shares Jupiter’s orbit in an asteroid swarm known as the Jupiter Trojans.
• The newly discovered trojan asteroid has been called an active asteroid.

About Active Asteroids

• Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics i.e., they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter’s orbit (like an asteroid).
• The first active asteroid discovered is 7968 Elst–Pizarro.
• Unlike comets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, active asteroids follow orbits within the orbit of Jupiter that are often indistinguishable from the orbits of standard asteroids.
• Some active asteroids show signs that they are icy in composition like a traditional comet, while others are known to be rocky like an asteroid.

About Trojan Asteroid

• The Jupiter trojans, commonly called Trojan asteroids or simply Trojans, are a large group of asteroids that share the planet Jupiter’s orbit around the Sun.

Relative to Jupiter, each Trojan librates around one of Jupiter’s two stable Lagrange points:

• L4, lying 60° ahead of the planet in its orbit, and
• L5, 60° behind.

Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.

• The Lagrangian points are the points near two large bodies in orbit where a smaller object will maintain its position relative to the large orbiting bodies.
• At other locations, a small object would go into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the two large bodies, the centripetal force of orbital motion, and (for certain points) the Coriolis acceleration all match up in a way that cause the small object to maintain a stable or nearly stable position relative to the large bodies.
• There are five such points, labeled L1 to L5, all in the orbital plane of the two large bodies, for each given combination of two orbital bodies.
• The first Jupiter trojan discovered, 588 Achilles, was spotted in 1906.
• A total of 7,040 Jupiter trojans have been found as of October 2018.

Difference between an asteroid and a comet

• The main difference between asteroids and comets is their composition, as in, what they are made of.
• Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material.
• Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago.
• Asteroids formed much closer to the Sun, where it was too warm for ices to remain solid. While Comets formed farther from the Sun where ices would not melt.
• Comets which approach the Sun lose material with each orbit because some of their ice melts and vaporizes to form a tail.
• Hence the tail of the comet that we see is nothing but the ice which melts and vaporizzes as the comet approaches the sun.